{"id":10613,"date":"2011-05-20T07:00:00","date_gmt":"2011-05-20T07:00:00","guid":{"rendered":"https:\/\/blogs.msdn.microsoft.com\/oldnewthing\/2011\/05\/20\/beginbufferedpaint-its-not-just-for-buffered-painting-any-more\/"},"modified":"2011-05-20T07:00:00","modified_gmt":"2011-05-20T07:00:00","slug":"beginbufferedpaint-its-not-just-for-buffered-painting-any-more","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20110520-00\/?p=10613","title":{"rendered":"BeginBufferedPaint: It's not just for buffered painting any more"},"content":{"rendered":"

\nI covered the BeginBufferedPaint<\/code> function in\n\nmy 2008 PDC presentation<\/a>,\nbut one thing I didn’t mention is that the buffered paint functions\nare very handy even if you have no intention of painting.\n<\/p>\n

\nSince the buffered paint functions maintain a cache\n(provided that you remembed to call\nBuffered­Paint­Init<\/code>),\nyou can use\nBegin­Buffered­Paint<\/code> to get a temporary bitmap\neven if you have no intention of actually painting to the screen.\nYou might want a bitmap to do some off-screen composition,\nor for some other temporary purpose,\nin which case you can ask\nBegin­Buffered­Paint<\/code> to give\nyou a bitmap,\nuse the bitmap for whatever you like,\nand then pass fUpdateTarget = FALSE<\/code> when you call\nEnd­Buffered­Paint<\/code>\nto say “Ha ha, just kidding.”\n<\/p>\n

\nOne thing to have to be aware of is that the bitmap provided\nby Begin­Buffered­Paint<\/code>\nis not guaranteed to be exactly\nthe size you requested; it only promises that the bitmap will be\nat least<\/i> the size you requested.\nMost of the time,\nyour code won’t care (there are just pixels out there that you aren’t\nusing),\nbut if you use the\nGet­Buffered­Paint­Bits<\/code>\nfunction to obtain direct access to the bits,\ndon’t forget to take the stride into account.\n<\/p>\n

Consider this artifical example of a program that uses\nCreate­DIB­Section<\/code> to create a temporary 32bpp bitmap\nfor the purpose of updating a layered window.\nStart with the\n\nscratch program<\/a>\nand make these changes:\n<\/p>\n

\nBOOL\nOnCreate(HWND hwnd, LPCREATESTRUCT lpcs)\n{\n BOOL fRc = FALSE;\n HDC hdcWin = GetDC(hwnd);\n if (hdcWin) {\n  HDC hdcMem = CreateCompatibleDC(hdcWin);\n  if (hdcMem) {\n   const int cx = 200;\n   const int cy = 200;\n   RECT rc = { 0, 0, cx, cy };\n   BITMAPINFO bmi = { 0 };\n   bmi.bmiHeader.biSize = sizeof(bmi.bmiHeader);\n   bmi.bmiHeader.biWidth = cx;\n   bmi.bmiHeader.biHeight = cy;\n   bmi.bmiHeader.biPlanes = 1;\n   bmi.bmiHeader.biBitCount = 32;\n   bmi.bmiHeader.biCompression = BI_RGB;\n   RGBQUAD *prgbBits;\n   HBITMAP hbm = CreateDIBSection(hdcWin, &bmi,\n             DIB_RGB_COLORS, &reinterpret_cast<void*&>(prgbBits),\n                                                        NULL, 0);\n   if (hbm) {\n    HBITMAP hbmPrev = SelectBitmap(hdcMem, hbm);\n    \/\/ Draw a simple picture\n    FillRect(hdcMem, &rc,\n                     reinterpret_cast<HBRUSH>(COLOR_INFOBK + 1));\n    rc.left = cx \/ 4;\n    rc.right -= rc.left;\n    rc.top = cy \/ 4;\n    rc.bottom -= rc.top;\n    FillRect(hdcMem, &rc,\n                   reinterpret_cast<HBRUSH>(COLOR_INFOTEXT + 1));\n    \/\/ Apply the alpha channel (and premultiply)\n    for (int y = 0; y < cy; y++) {\n     for (int x = 0; x < cx; x++) {\n      RGBQUAD *prgb = &prgbBits[y * cx + x];\n      BYTE bAlpha = static_cast<BYTE>(cx * x \/ cx);\n      prgb->rgbRed = static_cast<BYTE>(prgb->rgbRed * bAlpha \/ 255);\n      prgb->rgbBlue = static_cast<BYTE>(prgb->rgbBlue * bAlpha \/ 255);\n      prgb->rgbGreen = static_cast<BYTE>(prgb->rgbGreen * bAlpha \/ 255);\n      prgb->rgbReserved = bAlpha;\n     }\n    }\n    \/\/ update the layered window\n    POINT ptZero = { 0, 0 };\n    SIZE siz = { cx, cy };\n    BLENDFUNCTION bf =  { AC_SRC_OVER, 0, 255, AC_SRC_ALPHA };\n    fRc = UpdateLayeredWindow(hwnd, NULL, &ptZero, &siz, hdcMem,\n                              &ptZero, 0, &bf, ULW_ALPHA);\n    SelectBitmap(hdcMem, hbmPrev);\n    DeleteObject(hbm);\n   }\n   DeleteDC(hdcMem);\n  }\n  ReleaseDC(hwnd, hdcWin);\n }\n return fRc;<\/font>\n}\n<\/pre>\n
\nPretty standard stuff.\nBut let’s convert this to use the buffered paint functions\nto take advantage of the buffered paint bitmap cache.\n<\/p>\n
\nBOOL\nOnCreate(HWND hwnd, LPCREATESTRUCT lpcs)\n{\n BOOL fRc = FALSE;\n HDC hdcWin = GetDC(hwnd);\n if (hdcWin) {\n  HDC hdcMem;<\/font>\n  \/\/ HDC hdcMem = CreateCompatibleDC(hdcWin);<\/strike><\/font>\n  \/\/ if (hdcMem) {<\/strike><\/font>\n   const int cx = 200;\n   const int cy = 200;\n   RECT rc = { 0, 0, cx, cy };\n   \/\/ BITMAPINFO bmi = { 0 };<\/strike><\/font>\n   \/\/ bmi.bmiHeader.biSize = sizeof(bmi.bmiHeader);<\/strike><\/font>\n   \/\/ bmi.bmiHeader.biWidth = cx;<\/strike><\/font>\n   \/\/ bmi.bmiHeader.biHeight = cy;<\/strike><\/font>\n   \/\/ bmi.bmiHeader.biPlanes = 1;<\/strike><\/font>\n   \/\/ bmi.bmiHeader.biBitCount = 32;<\/strike><\/font>\n   \/\/ bmi.bmiHeader.biCompression = BI_RGB;<\/strike><\/font>\n   RGBQUAD *prgbBits;\n   BP_PAINTPARAMS params = { sizeof(params), BPPF_NOCLIP };\n   HPAINTBUFFER hbp = BeginBufferedPaint(hdcWin, &rc,\n                              BPBF_TOPDOWNDIB, &params, &hdcMem);\n   if (hbp) {\n    int cxRow;\n    if (SUCCEEDED(GetBufferedPaintBits(hpb, &prgbBits, &cxRow))) {<\/font>\n   \/\/ HBITMAP hbm = CreateDIBSection(hdcWin, &bmi,<\/strike><\/font>\n   \/\/        DIB_RGB_COLORS, &reinterpret_cast<void*&>(prgbBits),<\/strike><\/font>\n   \/\/                                                   NULL, 0);<\/strike><\/font>\n   \/\/ if (hbm) {<\/strike><\/font>\n    \/\/ HBITMAP hbmPrev = SelectBitmap(hdcMem, hbm);<\/strike><\/font>\n    \/\/ Draw a simple picture\n    FillRect(hdcMem, &rc,\n                     reinterpret_cast<HBRUSH>(COLOR_INFOBK + 1));\n    rc.left = cx \/ 4;\n    rc.right -= rc.left;\n    rc.top = cy \/ 4;\n    rc.bottom -= rc.top;\n    FillRect(hdcMem, &rc,\n                   reinterpret_cast<HBRUSH>(COLOR_INFOTEXT + 1));\n    \/\/ Apply the alpha channel (and premultiply)\n    for (int y = 0; y < cy; y++) {\n     for (int x = 0; x < cx; x++) {\n      RGBQUAD *prgb = &prgbBits[y * cxRow<\/font> + x];\n      BYTE bAlpha = static_cast<BYTE>(cx * x \/ cx);\n      prgb->rgbRed = static_cast<BYTE>(prgb->rgbRed * bAlpha \/ 255);\n      prgb->rgbBlue = static_cast<BYTE>(prgb->rgbBlue * bAlpha \/ 255);\n      prgb->rgbGreen = static_cast<BYTE>(prgb->rgbGreen * bAlpha \/ 255);\n      prgb->rgbReserved = bAlpha;\n     }\n    }\n    \/\/ update the layered window\n    POINT ptZero = { 0, 0 };\n    SIZE siz = { cx, cy };\n    BLENDFUNCTION bf =  { AC_SRC_OVER, 0, 255, AC_SRC_ALPHA };\n    fRc = UpdateLayeredWindow(hwnd, NULL, &ptZero, &siz, hdcMem,\n                              &ptZero, 0, &bf, ULW_ALPHA);\n    \/\/ SelectBitmap(hdcMem, hbmPrev);<\/strike><\/font>\n    \/\/ DeleteObject(hbm);<\/strike><\/font>\n   }\n   EndBufferedPaint(hpb, FALSE);<\/font>\n   \/\/ DeleteDC(hdcMem);<\/strike><\/font>\n  }\n  ReleaseDC(hwnd, hdcWin);\n }\n return fRc;<\/font>\n}\n\/\/ changes to WinMain\n if (SUCCEEDED(BufferedPaintInit())) {<\/font>\n \/\/ if (SUCCEEDED(CoInitialize(NULL))) {\/* In case we use COM *\/<\/strike><\/font>\n  hwnd = CreateWindowEx(WS_EX_LAYERED,<\/font>\n  \/\/ hwnd = CreateWindow(<\/strike><\/font>\n  ...\n  BufferedPaintUnInit();<\/font>\n  \/\/ CoUninitialize();<\/strike><\/font>\n  ...\n<\/pre>\n
\nWe’re using the buffered paint API not for buffered painting\nbut just as a convenient way to get a bitmap and a DC at one shot.\nIt saves some typing (you don’t have to create the bitmap and the DC\nand select the bitmap in and out),\nand when you return the paint buffer to the cache, some other\nwindow that calls Begin­Buffered­Paint<\/code> may be able\nto re-use that bitmap.\n<\/p>\n
\nThere are a few tricky parts here.\nFirst, if you’re going to be accessing\nthe bits directly, you need to call\nGet­Buffered­Paint­Bits<\/code>\nand use the cxRow<\/code> to determine the bitmap stride.\nNext, when we’re done, we pass FALSE<\/code> to\nEnd­Buffered­Paint<\/code> to say,\n“Yeah, um, thanks for the bitmap, but don’t Bit­Blt<\/code> the\nresults back into the DC we passed to\nBegin­Buffered­Paint<\/code>.\nSorry for the confusion.”\n<\/p>\n
\nA less obvious trick is that we used BPPF_NOCLIP<\/code>\nto get a full bitmap.\nBy default, Begin­Buffered­Paint<\/code>\nreturns you a bitmap which\nis clipped to the DC you pass as the first parameter.\nThis is an optimization to avoid allocating memory for pixels that\ncan’t be seen anyway when End­Buffered­Paint<\/code>\ngoes to copy\nthe bits back to the original DC.\nWe don’t want this optimization, however, since we have no intention\nof blitting the results back to the original DC.\nThe clip region of the original DC is irrelevant to us because\nwe just want a temporary bitmap for some internal calculations.\n<\/p>\n
\nAnyway, there you have it, an example of using\nBegin­Buffered­Paint<\/code> to obtain a temporary bitmap.\nIt doesn’t win much in this example (since we call it only once,\nat window creation time),\nbut if you have code which creates a lot of DIB sections for temporary\nuse, you can use this trick to take advantage of the buffered paint\ncache and reduce the overhead of bitmap creation and deletion.\n<\/p>\n
\nPre-emptive snarky comment<\/b>:\n“How dare you show us an alternative method that isn’t\navailable on Windows 2000<\/a>!”<\/p>\n","protected":false},"excerpt":{"rendered":"
I covered the BeginBufferedPaint function in my 2008 PDC presentation, but one thing I didn’t mention is that the buffered paint functions are very handy even if you have no intention of painting. Since the buffered paint functions maintain a cache (provided that you remembed to call Buffered­Paint­Init), you can use Begin­Buffered­Paint to get a […]<\/p>\n","protected":false},"author":1069,"featured_media":111744,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1],"tags":[25],"class_list":["post-10613","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
I covered the BeginBufferedPaint function in my 2008 PDC presentation, but one thing I didn’t mention is that the buffered paint functions are very handy even if you have no intention of painting. Since the buffered paint functions maintain a cache (provided that you remembed to call Buffered­Paint­Init), you can use Begin­Buffered­Paint to get a […]<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/10613","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/users\/1069"}],"replies":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/comments?post=10613"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/10613\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media\/111744"}],"wp:attachment":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media?parent=10613"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=10613"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=10613"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}